You just have to drift on the cliff. When you are on the cliff just click on the brake and just try to turn the car on the left or right, then you will be doing the flat spin.
Asphalt 8: Airborne is a 2013 racing video game, developed and published by Gameloft as part of the Asphalt series. It was released on August 22, 2013 for iOS and Android, and on November 13 for Windows 8 and Windows Phone 8.
If you want to download Asphalt 8(Airborne) for Android for free than you can click on the link below.
It is probably the most asked question while playing Asphalt 8. How to do a flat spin? Well here is the answer for that.